Tungsten carbide/porous carbon core-shell nanocomposites as a catalyst support for methanol oxidation

Article abstract of DOI:10.1039/c5ra18817b

Carbon-encapsulated tungsten carbide (WC@C) was prepared by a microwave-assisted synthesis method with resorcinol-formaldehyde resin (RF) as carbon source. WC was encapsulated by porous carbon layer to form core-shell structure which could protect tungsten oxide from occupying the active sites on the surfaces of the WC@C. The characteristics of WC@C composites were determined by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy and Brunauer-Emmet-Teller gas adsorption. Platinum nanoparticles were uniformly distributed on WC@C to synthesize a new electrocatalyst Pt-WC@C. The electro-catalytic performances of prepared Pt-WC@C, commercial Pt/C and PtRu/C toward methanol oxidation were compared by cyclic voltammetry, chronoamperometry and CO stripping test. It was found that Pt-WC@C exhibited higher catalytic activity for methanol oxidation than that of commercial Pt/C and PtRu/C catalysts. Especially, the Pt-WC@C achieved the long-term stability which was attributed to the effective protection by the carbon porous shell structure.

Full text of DOI:10.1039/c5ra18817b

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Tungsten carbide/porous carbon core-shell nanocomposites
as a catalyst support for methanol oxidation
Xiaoling Lang a, b, Meiqin Shia1*, Yekun Jianga, Huan Chena, Chunan Maa2†
a: State Key Laboratory Breeding Base for Green Chemistry Synthesis Technology, College of Chemical Engineering, Zhejiang University of Technology,
Hangzhou 310032, PR China
b: College of Chemical and Materials Science, Longyan University, Longyan 364000, PR China
Abstract
Carbonꢀencapsulated tungsten carbide (WC@C) was prepared by microwaveꢀassisted synthesis method with using
resorcinolꢀformaldehyde resin (RF) as carbon source. WC was encapsulated by porous carbon layer to form coreꢀ
shell structure which could protect tungsten oxide from occupying the active sites on the surfaces of the WC@C.
The characteristics of WC@C composites were determined by Xꢀray diffraction, scanning electron microscopy,
transmission electron microscopy, energy dispersive Xꢀray spectroscopy and BrunauerꢀEmmetꢀTeller gas
adsorption. Platinum nanoparticles were uniformly distributed on WC@C to synthesize a new electrocatalyst Ptꢀ
WC@C. The electroꢀcatalytic performances of prepared PtꢀWC@C, commercial Pt/C and PtRu/C toward methanol
oxidation were compared by cyclic voltammetry, chronoamperometry and CO stripping test. It was found that Ptꢀ
WC@C exhibited higher catalytic activity for methanol oxidation than that of commercial Pt/C and PtRu/C
catalysts. Especially, the PtꢀWC@C achieved the longꢀterm stability which was attributed to the effective protection
by the carbon porous shell structure.
Keywords: tungsten carbide, porous carbon, microwave, methanol oxidation, electroꢀcatalyst
1. Introduction
Direct methanol fuel cells (DMFC) are considered as an attractive future electroꢀcatalytic energy conversion
technology for power applications due to their high theoretical specific energy of methanol and low operating
temperature [1, 2]. Currently, the most effective electroꢀcatalyst for DMFC is the Pt metal or its alloy [3ꢀ5]. Although
these catalysts exhibit desirable electrochemical activities, both Pt and its alloy are expensive due to the scarce
world reserves of Pt. In addition, strong absorption of CO on the surface of Pt blocks the active sites for methanol
Corresponding author at: 1*+86057188320143; Eꢀmail: smq@zjut.edu.cn (meiqin Shi)
2† Tel:+86057188320830;
Fax: 86057188320830; Eꢀmail: science@zjut.edu.cn (chunan Ma)
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oxidation and results in loss of activity [6]. Consequently, the search for a lowꢀcost, highꢀactivity electroꢀcatalyst in
the anode compartment of DMFC is of growing interest as demand continues to rise. Although the Pt–Ru alloy
catalysts are expected to be the best candidate catalyst for DMFCs. However, their high cost still hinders fuel cells
commercialization and Ru dissolution from Pt–Ru catalysts is always a significant problem [7, 8]. Several approaches
have been attempted including loading small amount of Pt on nonꢀnoble metals [9ꢀ13]. Among the nonꢀnoble metals,
tungsten carbide (WC) has attracted attention from electrocatalysis researchers because it exhibits the better
synergistic effects with Pt [14ꢀ17]. WC surface is active toward the dissociation of H2O to produce surface hydroxyl
groups, which are propitious to the oxidation of the absorbed CO intermediate [18]. It is also found that WC could be
used as a support to increase the dispersion of the precious metal, and this WC based catalysts showed improved
activity for methanol oxidation reaction (MOR)[19, 20]. However, WC materials prepared by conventional methods
usually have high crystallinity and low surface area because of the high temperature in reduction and carburization
process, which weakens the combination with platinum [21]. The previous study suggests that another negative point
about WC was WC particles were thermodynamically unstable and easy to be oxidized in aqueous system at room
temperature. The resulted oxidized surface layer mainly consisting of WO3 covered the surface of WC catalysts and
became the barrier for the active sites [22]. The study about voltammetric behavior of a mixture of WC and Pt also
proved that soluble tungstate species, produced by chemical oxidation of WC, deposited on Pt in a form of hydrous
tungsten oxide and partially covered Pt surface [23]. In this case, the WC based catalysts can’t maintain their contents
and structures over the long time. Consequently this change in the surface layer of WC catalysts will result in a loss
of electrochemical activity. Herein, we intend to design a coreꢀshell structure to physically isolate WC catalysts
from the acid environment to avoid leaching, which however should not impede the catalytic properties of WC. This
design is inspired from the researches including Pd/C coreꢀshell nanocomposites for oxidation of alcohols, porous
Fe3O4/C coreꢀshell nanorods with good electromagnetic properties and other catalysts with similar coreꢀshell
features [24ꢀ26]. In these coreꢀshell systems direct contact of metal particles with harsh environments including acid
medium, oxygen, and sulfur contaminations is avoided. However, this protection does not impede the activation of
catalysts. On the contrary, the catalysts have a rather high activity and longꢀterm stability. This enhancement is
discussed by DFT calculations, indicating that the catalytic activity could arise from the electron transfer from metal
particles to the carbon shells leading to a decreased local work function on the carbon surface [27]
.
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In this work, we fabricated a novel support WC@C by a microwaveꢀassisted synthesis method with using
resorcinolꢀformaldehyde resin (RF) as the carbon source. Typically, carbonꢀbased nanomaterials have been used as
promising supports for low temperature fuel cells because carbonꢀbased material has the low electrical resistance to
facilitate electron transport during the electrochemical reactions [28].We design a carbon shell to inhabit or at least
deplete agglomeration of the particles, which can improve the dispersity of the WC particles. Moreover, the carbon
shell can protect WC from being oxidized in the electrolyte and thus increase their structure stability during cycling.
We also use a porous structure in the carbon shell for obtaining maximum contact with fuel and strong interaction
between catalyst and support.
Different from our previous work, in this study we used RF as in situ carbon source instead of the usual
external carbon source such as CO gas. And the prepared porous WC@C material by microwaveꢀassisted heating
has a uniform size distribution and stable microsphere structure. By loading a relatively small amount of Pt on the
WC@C microsphere we produced a new electrocatalyst for methanol oxidation which is far more active and stable
than the commercial Pt/C and PtRu/C catalysts.
2. Experimental
2.1. Catalyst preparation
A solution containing 2 ml of formaldehyde (37%) and 1g resorcinol was synthesized under vigorous stirring
at room temperature to make RF. Ammonium metatungstate (AMT, (NH4)6H2W12O40·xH2O) aqueous solution,
prepared by dissolving 2.5 g of AMT in 20 ml distilled water, was then added into the resorcinolꢀformaldehyde
solution with stirring for 30 min. Afterwards the mixed solution was heated by microwave system (Biotage, Initiator
EXP) at 85 ℃ for 10 min, then to 150 ℃and kept for 20 min. The asꢀmade products were dried at 80 ℃. All
chemicals used were of analytical grade.
The resulting precursor was carburized in a tube furnace system where was swept by nitrogen gas to remove
air. The precursor was heated to 400 ℃ at rate of 5 ℃ minꢀ1 and kept for 1 h , then the temperature was raised to
900 ℃, held for 4 h, and then cooled down to room temperature. The whole in situ reduction carbonization was
performed under argon and hydrogen without adding other carbon source. The obtained product was a black powder
denoted as WC@C. The formation process of the WC@C composite is schematized in Fig. 1. For comparison, WC
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was synthesized by the same method, but the reduction carburization was completed under CO atmosphere. The
noble metal Pt (10 wt %) particles were loaded on the WC@C by microwave heating according to an established
method [29]. The product was marked as PtꢀWC@C, and the 20% PtꢀWC@C (Pt 20 wt %) material was prepared by
the same method for comparison.
Figure 1.Schematic illustrating the method to use microwaveꢀassisted synthesis to form WC@C coreꢀshell
nanocomposites.
2.2. Catalyst characterization
The morphology and structure of catalysts were characterized by transmission electron microscopy (TEM) and
scanning transmission electron microscope (STEM) using a Tecnai G2 F30 SꢀTwin microscope (FEI, Netherlands)
at a voltage of 200 kV and a current of 103 mA, coupled with energy dispersive XꢀRay spectrometer (EDX, Thermo
NORAN VANSTAGE ESI). The morphologies of the materials were also observed by scanning electron
microscopy (SEM, Hitachi Sꢀ4700Ⅱ) using an accelerating voltage of 3.0 kV. The Xꢀray diffraction patterns (XRD,
Thermo ARL SCINAG X TRA) were used to characterize the crystalline structure of the products with Cu Kα source
operated at 45 kV. N2 adsorption–desorption was examined by ASAP 2020 (Micromeritics, America) and the
specific surface area of WC@C was measured by Brunauer–Emmett–Teller (BET) method. Raman spectra were
obtained at room temperature by Raman spectrometer (JOBIN YVON Lab RAM HR UV800) at an excitation
wavelength of 514.5 nm.
2.3. Electrochemical measurements
The electrochemical measurements were carried out by CHI 660C electrochemical workstation in a standard
threeꢀelectrode cell. A 3 mm diameter glass carbon electrode (GC) deposited with catalyst was used as the working
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electrode, Pt foil and saturated calomel electrode (SCE) were used as counter electrode and reference electrode
respectively. 5 mg PtꢀWC@C sample was dispersed in 0.1 ml isopropyl alcohol and 20 µL Nafion (5 wt%, Du Pont
Corp. USA) under ultrasonic treatment for 10 min. Then the ink was deposited on the surface of GCꢀelectrode and
dried at 60 ℃to remove the solvent. The Pt loading of the all electrodes is 8.3 µg.
The cyclic voltammetry (CV) curves were measured in 0.5 M H2SO4 with 0.5 M CH3OH at a scan rate 50 mV
sꢀ1. The chronoamperometry test (CA) was performed in the same electrolyte 0.5 M H2SO4 containing 0.5 M
CH3OH at potential of 0.2 V. For the CO stripping voltammetry, CO was bubbled into the 0.5 M H2SO4 for 30 min
while keeping the potential at −0.14 V after the electrolyte was purged with nitrogen for 30 min. Then the nitrogen
was purged again for 30 min to remove trace of dissolved CO. Finally the CO stripping voltammetry was measured
from −0.2 to 1.0 V at a scan rate 50 mV sꢀ1 at room temperature. The other electrochemistry experiments were
carried out at 50 ± 0.5 ℃. Both the CV and CA were obtained after the electrolyte was purged with high purity
nitrogen for 20 min. The commercial Pt/C (20% Pt on Vulcan XCꢀ72R, Johnson Matthey Corp.) and PtRu/C catalyst
(10% Pt: 10%Ru on Vulcan XCꢀ72R, Johnson Matthey Corp.) were also tested for comparison.
3. Results and discussion
Cyclic voltammograms of WC@C and pure WC recorded in 0.5 M H2SO4 at a scan rate 50 mV sꢀ1 from ꢀ0.2 to
1.0 V is given in Fig. 2. The voltammogram of the WC@C powder is stable over time while an anodic peak is
observed at 0.8 V on pure WC. This anodic peak represents the voltammetric behavior of WO3, which indicates that
the WC surface is stable in its oxidized state. The oxidation of WC induced by anodic polarization has been reported
by Lee et al. [30]. They found the similar result that a sharp anodic peak at 0.8 V could be observed for pure WC in
aqueous solution. In the presence of water and/or oxygen, WC is oxidized to surface oxide and/or soluble W (VI)
species via the following reactions: [31, 32]
WC + 5H2O → WO3 + CO2 + 10H+ + 10 e–
WC + 6H2O → WO42– + CO2 + 12H+ + 10 e–
(1)
(2)
Different from the pure WC, no anodic peak can be observed on WC@C in the whole scan range suggesting
the carbon shell encapsulated on WC can prevent the WC from further oxidation.
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Figure 2.Cyclic voltammograms of WC and WC@C in 0.5 M H2SO4 at 50 mV sꢀ1 from ꢀ0.2 to 1.0 V.
The degree of graphitization of the carbon in WC@C composite was investigated by Raman spectra, shown in
Fig.3. The spectra show two Raman characteristic peaks at 1310 and 1590 cmꢀ1. The peak at around 1310 cmꢀ1, be
named the Dꢀband, can be attributed to the presence of sp3 defects within the carbon. The Dꢀband is associated with
the A1g vibration of disordered carbon atoms. The peak at 1590 cmꢀ1, named the Gꢀband, corresponds to vibration of
graphitic carbon atoms. The Gꢀband is associated with the Ramanꢀactive E2g mode. The intensity ratio of the Dꢀband
to Gꢀband (I
/ I Gꢀband) is indicative of the graphite site density [33]. The ratio value of I
/ I for the
Gꢀband
Dꢀband
Dꢀband
prepared WC@C is 1.182, implying a microꢀordered graphitic structure for carbon shells.
Figure 3.Raman spectra of WC@C material.
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Fig. 4 displays the XRD patterns of the WC@C and PtꢀWC@C samples. It can be found that the sample
WC@C is mainly composed of hexagonal WC phase whose main peaks could be clearly identified according to the
JCPDS card No.00ꢀ002ꢀ1055, suggesting that most of tungsten source AMT has been inꢀsitu carbonized to WC in
the presence of the resorcinol formaldehyde resin. The diffraction peaks at 31.507°, 35.550°and 48.255°can be
assigned to the crystal planes of WC (001), (100) and (101), respectively. Some diffraction peaks of W2C are also
detected the WC@C sample. The diffraction peaks at 39.939°, 46.176°and 67.660°correspond to the crystal planes
of faceꢀcenteredꢀcubic (f. c. c.) platinum cell (111), (200) and (220). In addition, the average particle size of Pt
calculated using the DebyeꢀScherer equation [34] from the plane (200) is 5.18 nm. No characteristic peak of graphiteꢀ
like carbon is observed at 2θ = 25.5°. This absence of the graphitic structure implies that only amorphous carbon
aerogel can be formed from resorcinol formaldehyde resin.
Figure 4.XRD patterns of the WC@C and PtꢀWC@C particles.
The structural and morphological characteristics of WC@C samples were observed by SEM and TEM, as
shown in Fig. 5. It can be found from Fig. 5a and b, the microsphere appears to be an aggregate of small WC
particles that are wrapped with an amorphous carbon shell. The same results are also proved by TEM images. In Fig.
5c and d, WC@C particles consist of a core of WC (black area) and a shell of carbon (gray area) which confirms the
metal WC particles are encapsulated by the carbon shell. Fig. 6 shows the highꢀresolution transmission electron
microscope (HRTEM) images of WC particles. The dꢀspacings of the core structure are 0.125 and 0.145 nm,
corresponding to WC (102) and (110) planes. No obvious lattice pattern of the carbon shell is observed which
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explains the absence of the graphitic structure in the XRD patterns. The EDS analysis (Fig. 6) for the shell indicates
the single element of carbon while the elements of the core consist of W and C which also testifies the formation of
coreꢀ shell structure.
Figure 5. SEM (a, b) and TEM (c, d) images of WC@C particles.
Figure 6.HRTEM image of WC@C particle and EDS patterns of (a) Part a and (b) Part b in the HRTEM image.
The possible mechanism for the synthesis process is schematized in Fig. 1. It is well known that resorcinol and
formaldehyde can form a Bakelite type polymer [35].In our study, initially a mixture was formed when adding
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resorcinol, formaldehyde and AMT in the aqueous solution under stirring. After the condensation polymerization a
crossꢀlinking polymer network was fabricated within microwave heating system where offered a hermetic and highꢀ
pressure environment. During this polymerization process, AMT immersed in the crossꢀlinking network and was
entrapped in the RF. Then the polymer composite was heated at high temperature under argon and hydrogen flow.
The entrapped AMT was reduced and carburized to WC, and part of the polymer took the role as the carburized
source to form WC, and the other contributed to the carbonꢀencapsulated shell. In this way, the uniformly dispersed
WC@C particles were synthesized after carbonization.
The platinum particles were loaded on WC@C by the microwave heating polylol reduction method in alkaline
media. Both TEM and HRTEM images in Fig. 7 show that Pt nanoparticles are well supported on WC@C. This
designed structure is supposed to affect the stability of catalytic reaction due to the improved interaction between
active component and support. The HRTEM image exhibits the lattice fringe spacing of Pt is about 0.139 nm, which
corresponds to (220) plane. The histograms show that the particle size of Pt is about 5 nm in diameter, which agree
with the calculation result of Debye–Scherrer equation in XRD.
Figure 7.TEM and HRTEM images of PtꢀWC@C catalyst, and histograms of the particle size distributions of Pt particles
about the PtꢀWC@C catalysts.
Fig. 8A gives the N2 adsorption/desorption isotherm of the WC@C sample. With pressure increasing, the
isotherm shows a sharp increase in the amount of adsorption followed by saturation, and there is a hysteresis
between adsorption and desorption branches. This typical Type Ⅳ represents characteristics of mesoporous
materials. The BET surface area of WC@C microspheres calculated from data at the lower N2 pressures is about
270 m2 gꢀ1. The mesoporous structure is also confirmed from the smallꢀangle XRD pattern (Fig. 8B). The sample
WC@C shows reflection clearly at 2
θ
= 1° which suggests the existence of ordered mesopores on surface of the
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sample. This porous structure was formed when RF networks were filled with hydrogen gas and then fully removed
during the carbonization.
Figure 8. (A) The nitrogen adsorption/desorption isotherm of WC@C microspheres. (B) Smallꢀangle Xꢀray scattering
pattern of WC@C particles.
Fig.9 presents the cyclic voltammograms of commercial Pt/C, PtRu/C and PtꢀWC@C in 0.5 M H2SO4
solution. All the catalysts clearly show the hydrogen adsorption and desorption peaks on platinum surface at the
potentials from ꢀ0.2 V to 0.1 V and the PtꢀO formation at the potential of 0.4 V ꢀ 0.5 V in the backward sweep.
These typical regions demonstrate the presence of polycrystalline Pt. It can be noticed that PtꢀWC@C has higher
current peak for the hydrogen adsorption/desorption than that of commercial Pt/C and PtRu/C. This is the evidence
for explaining the spillover effects between Pt and WC. Spillover of hydrogen from a metal to an oxide or carbon
surface is well studied for many catalytic reactions involve hydrogen because most metal catalysts consist of small
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metal particles supported on either high surface area oxides or carbon [36, 37]. For example, the synergistic effect
between WO3 and Pt relies on formation of tungsten bronzes by the Faradaic process of intercalation/deꢀ
intercalation of H atoms into WO3, which is spillover of hydrogen, showing as following:
（3）
However, in our case, spillover of hydrogen happens from Pt to WC surfaces as well as carbon shell. The
similar phenomenon was discussed in the research work about Pt/WC [38ꢀ40]. Not like oxide and carbon, the process
and mechanism of H+ spillover from Pt to WC was not fully understood. An accepted explanation discussed that the
much higher ECSA value for the Pt/WC catalyst could be accounted for by active participation of WC in
electrochemical hydrogen oxidation together with Pt. Another synergistic effect between WC and Pt was also
discussed. Since the WC can also absorb OHads, thereby finally relieve more Pt active sites by facilitating the
oxidation of COads and other intermediates which cover the PtꢀWC@C interface. Thus, we can find a fine correlation
between the activity of methanol oxidation and ECSA values of the catalysts.
Figure 9.Cyclic voltammograms of commercial Pt/C, PtRu/C and PtꢀWC@C catalysts in 0.5 M H2SO4 electrolyte.
The electrocatalytic properties for methanol electrooxidation of commercial Pt/C、PtRu/C and PtꢀWC@C
catalysts were compared by CVs measurement in 0.5 M H2SO4 + 0.5 M CH3OH in Fig. 10. As shown in Fig. 10, Ptꢀ
WC@C shows higher peak current density compared with commercial Pt/C and PtRu/C for methanol oxidation. In
the methanol electrooxidation, the first anodic peak is usually assigned to the oxidation of methanol molecules on
the electrode surface, and the backward anodic peak is generally attributed to the continuous oxidation of
incompletely oxidized carbonaceous intermediates accumulated on the catalyst surface during the forward scan,
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such as COads, COOHads, and COHads [41]. The ratio of the forward anodic peak current density (If) to the reverse
anodic peak current density (Ib), i.e., If/Ib, can be used to describe the catalyst tolerance to carbonaceous species
accumulation. The high If/Ib ratio indicates excellent oxidation of methanol during the reverse anodic scan and less
accumulation of residues on the catalyst [42].The ratios of the forward anodic peak current density (If) to the reverse
anodic peak current density (Ib), i.e., If/Ib on PtꢀWC@C is much higher than that on commercial Pt/C and PtRu/C
(see Table 1). It is meaningful for us to notice that the PtꢀWC@C catalyst gives larger If/Ib value (1.48) than that on
commercial Pt/C (0.88) and PtRu/C (1.11), which means that the Pt is better dispersed on the WC@C support can be
much more tolerant to the intermediate carbon species and consequently facilitate the oxidization of methanol to
carbon dioxide much more efficient. The above higher catalytic activity demonstrated by the PtꢀWC@C catalyst can
be ascribed to the larger ECSA and uniform small size Pt, especially the synergistic effect between WC and Pt.
In order to demonstrate the Pt particles can be better dispersed on the WC@C support, we compared the
catalytic activities and TEM images of the commercial Pt/C and 20%PtꢀWC@C catalysts (see Fig. 11). The higher
catalytic activities displayed by the 20%PtꢀWC@C catalyst can be ascribed to the better dispersed Pt on the novel
support WC (see Fig. 11(a) (b)). It is show that 20%Pt can be also better dispersed on the WC@C support.
Figure 10 Cyclic voltammograms of commercial Pt/C、PtRu/C and PtꢀWC@C catalysts in 0.5 M H2SO4 and 0.5 M
CH3OH electrolyte.
Table 1 Comparison of different parameters between Pt/C、PtRu/C and PtꢀWC@C catalysts.
Samples
PtꢀWC@C
PtRu/C
If(A/g Pt)
509.94
389.17
263.76
Ib(A/g Pt)
343.71
If/Ib
1.48
1.11
0.88
351.85
commercial Pt/C
299.76
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Figure 11, TEM images of commercial Pt/C (a), 20% PtꢀWC@C (b), and (c) Cyclic voltammograms of commercial Pt/C
and 20%PtꢀWC@C catalysts in 0.5 M H2SO4 and 0.5 M CH3OH electrolyte
The CO tolerance of the prepared electrocatalyst can be evaluated by the CO stripping experiment as shown in
Fig. 12. Before the stripping experiment, electrodes were immersed in 0.5 M H2SO4 solution where was bubbled
with CO for 30 min and then purged with high pure N2 for 30 min. Compared with the other samples, no obvious
CO absorptionꢀstripping peak can be observed on the support WC@C (inset in Fig.12). One possible reason is that
CO is relatively weakly bonded on WC@C [43]. The experimental CO uptake of WC samples was around 7 mmol gꢀ
1, which been several orders of magnitude less than that of Pt metal [44]. CO adsorption experiments of water and CO
on C/W (111) show that the presence of surface hydroxyls hinders the adsorption of CO. Thus, even after
coadsorbing water and CO on the WC@C surface, only trace amounts of CO can be oxidized, which is not
detectable. Compared with commercial Pt/C, PtRu/C and PtꢀWC@C catalysts for CO oxidation, the PtꢀWC@C
could have the higher activity for CO oxidation is the evidence of promoting effect of WC in terms of H2O
dissociation and CO oxidation [43, 45], implying that the poisoningꢀtolerance and performance of the DMFC could be
improved in the involvement of WC.
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Figure 12.The CO stripping data on WC@C (inset), commercial Pt/C, PtRu/C and PtꢀWC@C catalysts in 0.5 M H2SO4
aqueous solution.
To evaluate the stability of PtꢀWC@C catalyst for methanol oxidation, the Chronoamperometry test was
recorded and compared with commercial Pt/C and PtRu/C catalyst as shown in Fig. 13. In the initial period, the
potentiostatic current decreases rapidly for both two catalysts, perhaps due to charge with the electrical double layer.
After 1000 s, the current densities of the three catalysts are stable, but the PtꢀWC@C maintains at a higher steady
state than the commercial Pt/C, PtRu/C because WC can activate water and form surface hydroxyls easily, and
hence enhance the CO tolerance of the catalysts. Moreover, the carbon shell could extend the function of WC even
in the long cycles of reaction as it can keep the activity of WC from being oxidized.
Concerning the further work, the DMFC performances of prepared electrocatalysts will be evaluated by
confirming the halfꢀcell results.
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Figure 13.Chronoamperometry curves for commercial Pt/C, PtRu/C and PtꢀWC@C catalysts in 0.5M CH3OH and 0.5 M
H2SO4 aqueous solution at potential of 0.6 V.
4. Conclusion
In this study, the WC@C support was prepared by microwaveꢀassisted synthesis method with using RF as in
situ carbon source. The electroꢀoxidation of methanol and CO stripping test show that Ptꢀ WC@C catalyst has the
best performance for methanol oxidation and CO tolerance compared with those of commercial Pt/C and PtRu/C
catalysts. It is that WC is encapsulated by porous carbon shell structure. This porous carbon shell structure can
suppress the agglomeration of catalysts and provide more interfaces between different components. More important,
this carbon shell structure retards the oxidation of WC surface and prohibits tungsten oxide from occupying the
active sites on the surfaces of the PtꢀWC@C catalysts. Therefore, more active sites remain and the synergistic effect
of Pt and WC is improved, finally results in the enhanced activity and stability.
5. Acknowledgements
This research was funded by International Science ＆ Technology Cooperation Program of China
(2010DFB63680) and National Natural Science Foundation of China (21376220), partially supported by Zhejiang
Provincial Natural Science Foundation of China(LY16B060009) and Science and Technology Key Project of Fujian
Province (2014H0038).
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The Pt-WC@C demonstrates higher electrochemical activity, which could be
attributed to the better dispersed Pt on WC which leads to the improved synergistic
effect between WC and Pt.